Driving apparatus for vehicles

ABSTRACT

A driving apparatus is used in a vehicle wherein both the front and rear wheels are driven. The apparatus prevents the steerable wheels from skidding while transmitting power to the steerable wheels when turning the vehicle. The apparatus includes a transmission unit  13  for receiving a rotational output from a main HST  50,  a steerable wheel drive shaft  6  and a nonsteerable wheel drive shaft  5  for receiving the rotational output from the transmission unit and delivering the output respectively to an axle for driving the steerable wheels and an axle for driving the nonsteerable wheels, and a differential unit for rotating the drive shaft  6  at an increased speed and the drive shaft  5  at a decreased speed.

DETAILED DESCRIPTION OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a driving apparatus for use infour-wheel drive vehicles for transmitting power from a drive source tosteerable wheels and nonsteerable wheels via a main HST (hydraustatictransmission).

2. Prior Art

When four-wheel drive vehicles having steerable wheels and nonsteerablewheels respectively at a front position and a rear position of thevehicle are turned, a difference occurs in turning radius between thesteerable wheel and the nonsteerable wheel. For example, when thevehicle shown in FIG. 1 and having rear wheels serving for steering isturned, the rear wheel turns along a path of a greater radius than thefront wheel. If the front and rear wheels are rotated at the same speed,therefore, the rear wheel skids on the terrain. Such a skid of the rearwheel roughs the terrain or causes wear on the rear wheel. Especiallywith vehicles adapted to travel on lawns like lawn mowers, roughing theterrain is undesirable.

Already proposed to preclude this drawback are (1) a driving apparatushaving a one-way clutch provided in the path of power transmission tothe steerable wheels for transmitting a driving power to the steerablewheel and also permitting the steerable wheel to rotate at a higherspeed than the nonsteerable wheel when the vehicle is turned, and (2) adriving apparatus wherein the path of power transmission to thesteerable wheels is provided with a gear transmission unit for giving anincreased speed. The gear transmission unit is brought out of operationwhen the vehicle is advanced straight, or is operated to rotate thesteerable wheel at a higher speed than the nonsteerable wheel when thevehicle is turned.

However, the driving apparatus (1) has the drawback the vehicle fails tofully exhibit the performance of four-wheel drive since the nonsteerablewheels only are driven as in a two-wheel drive vehicle when the vehicleis turned. This entails another drawback that the vehicle is impaired inrunning performance when turning with a small radius since thenonsteerable wheels only are driven for turning.

Further with the driving apparatus (2), the steerable wheel is rotatableonly at two speeds, i.e., a high speed and the same speed as thenonsteerable wheel. On the other hand, the difference between thesteerable wheel and the nonsteerable wheel in turning radius isproportional to the steering angle of the steering wheel to be handledby a driver. Thus, in order to effectively prevent the steerable wheelsfrom skidding during turning of the vehicle, there is a need to vary therotational speed difference between the steerable wheel and thenonsteerable wheel. Accordingly the driving apparatus (2) which affordsonly two different rotational speeds to the steerable wheels fails tofully preclude the steerable wheels from skidding. Further although thedriving apparatus is capable of providing two different speeds, i.e., ahigh speed and the same speed, for the steerable wheel, it is impossibleto increase the rotational speed difference between the steerable wheeland the nonsteerable wheel because the rotational speed of thenonsteerable wheel is constant. If the vehicle is turned with a smallradius, i.e., if the difference between the steerable wheel and the nonand the nonsteerable wheel in turning radius is great, it is difficultto obtain such a speed difference as to offset the radius difference.

Also known is a driving apparatus similar to the apparatus 1 and havinga transmission unit for giving an increased speed which unit comprises apair of pulleys and a belt reeved around the pulleys, the pulleys beingvariable in effective diameter according to the rotation angle of thesteering wheel at the driver's seat. However, although adapted to varythe rotational speed of the steerable wheel, the apparatus alsoencounters difficulty in increasing the rotational speed differencebetween the steerable wheel and the nonsteerable wheel since therotational speed of the nonsteerable wheel is constant.

Further, in a vehicle with a four-wheels steering system, the resemblingproblem occurs. That is, the turning radius of front ground wheel andrear ground wheel is usually different to each other. Therefore, theadjustment between the rotational speed of the front wheels and the rearwheels is necessary.

SUMMARY OF THE INVENTION

An object of the present invention, which has been accomplished toovercome the foregoing problems, is to provide a driving apparatus foruse in four-wheel drive vehicles, the apparatus being adapted toeffectively prevent the steerable wheels from skidding whiletransmitting power to the steerable wheels when turning the vehicle.

To fulfill the foregoing object, the present invention provides adriving apparatus for a vehicle for transmitting power from a drivesource installed in a body of the vehicle to steerable wheels andnonsteerable wheels via a main HST, the driving apparatus comprises: atransmission unit for receiving a rotational output from the main HST, asteerable wheel drive shaft and a nonsteerable wheel drive shaft forreceiving a rotational output from the transmission unit andtransmitting the rotational output respectively to an axle for drivingthe steerable wheels and an axle for driving the nonsteerable wheels,and a differential unit for rotating the steerable wheel drive shaft atan increased speed and rotating the nonsteerable wheel drive shaft at adecreased speed according to the steering angle of the steering wheel.

Preferably, the driving apparatus can be so constructed that thesteerable wheel drive shaft and the nonsteerable wheel drive shaft arearranged on approximately the same axis and spaced apart from each otherat opposed ends thereof, the transmission unit comprising: a main driveshaft disposed between the opposed ends of the steerable wheel driveshaft and the nonsteerable wheel drive shaft on the same axis as the twoshafts, a driving power transmission mechanism for transmitting therotational output of the main HST to the main drive shaft, and asteerable wheel planetary gear unit and a nonsteerable wheel planetarygear unit for transmitting the rotation of the main drive shaftrespectively to the steerable wheel drive shaft and the nonsteerablewheel drive shaft so as to rotate the wheel drive shafts in the samedirection, the steerable wheel planetary gear unit having a first sungear mounted on the main drive shaft, a first outer wheel surroundingthe first sun gear, a first inner gear provided on an inner periphery ofthe first outer wheel, first planetary gears arranged between the firstsun gear and the first inner gear, and a first carrier supported on thesteerable wheel drive shaft nonrotatably relative thereto and rotatablewith the revolution of the planetary gears, the nonsteerable wheelplanetary gear unit having a second sun gear mounted on the main driveshaft, a second outer wheel surrounding the second sun gear, a secondinner gear provided on an inner periphery of the second outer wheel,second planetary gears arranged between the second sun gear and thesecond inner gear, and a second carrier supported on the nonsteerablewheel drive shaft nonrotatably relative thereto and rotatable with therevolution of the second planetary gears.

The driving apparatus can be so constructed that the differential unitcomprises: a differential HST for receiving the power from the drivesource and outputting a differential rotational drive force, and asecond differential power transmission mechanism for receiving therotational output from the differential HST and giving the first outerwheel and the second outer wheel respective additional rotations inopposite directions to each other, the differential HST being adaptednot to output the rotational drive force when the vehicle is advancedstraight and to output the rotational drive force with a number ofrevolutions in accordance with the steering angle of the steering wheelwhen the vehicle is turned, the differential power transmissionmechanism being adapted to give the first outer wheel an additionalrotation of the same direction as the rotation of the first sun gearrotated by the main HST and to give the second outer wheel an additionalrotation opposite in direction to the rotation of the second sun gearrotated by the main HST.

Further the driving apparatus can be so constructed that thedifferential HST has a hydraulic pump and a hydraulic motor, at leastone of the hydraulic pump and the hydraulic motor being of the variabledisplacement type which is variable in displacement by operating adisplacement altering member, the differential unit comprising a linkmechanism coupling the steering wheel to the displacement alteringmember, the link mechanism being adapted to reduce approximately to zerothe discharge rate of one of the hydraulic pump and the hydraulic motorwhich has the displacement altering member when the steering wheel is ina posture to advance the vehicle straight and to increase the dischargerate as the steering angle of the steering wheel increases when thesteering wheel is in a posture to turn the vehicle.

Alternatively, the driving apparatus can be so constructed that thesteerable wheel drive shaft and the nonsteerable wheel drive shaft arearranged on approximately the same axis and spaced apart from each otherat opposed ends thereof, the transmission unit comprising a firstdifferential gear mechanism having a pair of first sun gears fixedlymounted on the steerable wheel drive shaft and the nonsteerable wheeldrive shaft, respectively, at their opposed ends, the first differentialgear mechanism comprising a first casing covering the pair of first sungears and rotatably supported on the drive shafts, a first ring gearprovided externally on the first casing for receiving the rotationaloutput from the main HST, and a plurality of first planetary gearsmeshing with the pair of first sun gears and rotatably supported by ashaft fixedly provided inside the first casing and extendingdiametrically of the casing, the differential unit comprising adifferential HST for receiving the power from the drive source andoutputting a differential rotational drive force, a second differentialgear mechanism for receiving the rotational output from the differentialHST, and an adjusting transmission mechanism for transmitting an outputfrom the second differential gear mechanism to the steerable wheel driveshaft and the nonsteerable wheel drive shaft, the second differentialgear mechanism comprising a pair of rotary shafts supported by a housingof the driving apparatus, arranged on approximately the same axis andspaced apart from each other at opposed ends thereof, a pair of secondsun gears fixedly mounted on the respective rotary shafts at theiropposed ends, a second casing covering the pair of second sun gears androtatably supported on the pair of rotary shafts, a second ring gearprovided externally on the second casing for receiving the rotationaloutput from the differential HST, and a plurality of second planetarygears meshing with the pair of second sun gears and rotatably supportedby a shaft fixedly provided inside the second casing and extendingdiametrically of the second casing, the adjusting transmission mechanismcomprising two gear trains provided for the steerable wheel drive shaftand the nonsteerable wheel drive shaft respectively and each comprisinga drive gear fixed to the rotary shaft, and a driven gear fixed to thecorresponding drive shaft, at least one of the two gear trains having atleast one intermediate gear so as to render the two driven gearsrotatable in directions different from each other, the drive gear andthe driven gear of each of the gear trains being equal in diameter.

Preferably, each of the driving power transmission mechanism and thedifferential power transmission mechanism can be a power transmissiongear mechanism.

As explained above, the driving apparatus embodying the invention foruse in vehicles comprises a transmission unit for receiving a rotationaloutput from a main HST, a steerable wheel drive shaft and a nonsteerablewheel drive shaft for receiving a rotational output from thetransmission unit, and a differential unit for rotating the steerablewheel drive shaft at an increased speed and rotating the nonsteerablewheel drive shaft at a decreased speed according to the steering angleof the steering wheel of the vehicle, so that when the vehicle isturned, the steerable wheels can be effectively precluded from skiddingwith the vehicle propelled by four-wheel drive.

The transmission unit comprises a main drive shaft for receiving therotational output of the main HST via a driving power transmissionmechanism, and a steerable wheel planetary gear unit and a nonsteerablewheel planetary gear unit for transmitting the rotation of the maindrive shaft respectively to the steerable wheel drive shaft and thenonsteerable wheel drive shaft; and the differential unit comprises adifferential HST for outputting a rotational drive force in accordancewith the steering angle of the steering wheel, and a driving powertransmission mechanism for receiving the rotational output of thedifferential HST, giving the steerable wheel drive shaft an additionalrotation for a speed increase and giving the nonsteerable wheel driveshaft an additional rotation for a speed reduction. When the drivingapparatus is thus constructed, the rotational speed of the steerablewheels and the nonsteerable wheels can be controlled with good stabilityfor turning the vehicle.

Further when the differential HST is of the variable displacement typehaving a movable swash plate or like displacement altering member, andthe steering wheel is operatively connected to the displacement alteringmember by a link mechanism, the steering angle of the steering wheel canbe operatively related to the rotational output of the differential HSTwith good stability.

When the driving power transmission mechanism and the differential powertransmission mechanism are each a power transmission gear mechanism, animproved transmission efficiency can be achieved.

Another aspect of the present invention provides a driving apparatus fora vehicle for transmitting power via a main HST from a drive sourceinstalled in a body of the vehicle to a first pair of driving wheels anda second pair of driving wheels, the pairs of wheels being positioned atthe front and rear of the vehicle body and at least one pair of thepairs of driving wheels being steerable, the driving apparatus beingcharacterized in that the driving apparatus comprises: a transmissionunit for receiving a rotational output from the main HST, a first driveshaft and a second drive shaft for receiving a rotational output fromthe transmission unit and transmitting the rotational outputrespectively to an axle for driving the first pair of wheels and an axlefor driving the second pair of wheels, and a differential unit forrotating the first drive shaft and the second drive shaft at a speedadjusted according to the steering angle of a steering wheel to behandled by a driver.

Therefore, for a vehicle with a four-wheels steering system, i.e. for avehicle with a first pair of driving steerable wheels and a second pairof driving steerable wheels positioned at the front and rear of thevehicle body, the differential unit can be so structured that thedifferential unit rotates the first drive shaft and the second driveshaft at a speed adjusted according to the steering angle of a steeringwheel to be handled by a driver. According to so structured drivingapparatus, the pair of the front wheels and the pair of the rear wheelsare driven by the first and second drive shafts rotating with adjustedspeed according to the steering angle of the steering wheel, and thusprecluded from skidding even when the difference between the turningradius of front wheel and rear wheel occurs with turning of a vehicle.

The invention will be further clarified by the description ofembodiments with reference to the following accompanying drawings. Theinvention is not limited to these embodiments, but various modificationsare possible without deviation from the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary plan view of a vehicle comprising a preferredembodiment of driving apparatus of the invention.

FIG. 2 is a plan view in section of a main HST included in the vehicleof FIG. 1 and the vicinity thereof.

FIG. 3 is a cross sectional view in development showing a front driveshaft, rear drive shaft and transmission unit included in the vehicle ofFIG. 1 and the vicinity of these components.

FIG. 4 is a view in section taken along the line A—A in FIG. 1.

FIG. 5 is a development in vertical section of a differential unitincluded in the vehicle of FIG. 1 and the vicinity thereof.

FIG. 6 is a diagram showing the steering wheel of the vehicle of FIG. 1and the rear wheels thereof as coupled thereto.

FIG. 7 is a diagram showing a link mechanism of the driving apparatusaccording to the embodiment when the vehicle is advanced straight.

FIG. 8 is a diagram showing the link mechanism of FIG. 7 when thevehicle is turned counterclockwise.

FIG. 9 is a diagram showing the link mechanism of FIG. 7 when thevehicle is turned clockwise.

FIG. 10 is a diagram showing the power transmission system of thevehicle of FIG. 1.

FIG. 11 is a diagram showing differences in turning radius between thesteerable wheels and nonsteerable wheels of the vehicle of FIG. 1.

FIG. 12 is a diagram showing a power transmission system of a vehiclemodified from the system shown in FIG. 10.

PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of driving apparatus of the present inventionwill be described below with reference to the drawings. FIG. 1 is afragmentary plan view of a vehicle 100 comprising a driving apparatus 1embodying the invention. The driving apparatus of the embodiment isadapted for use in four-wheel drive vehicles wherein power is deliveredfrom a drive source 60 installed in the body of the vehicle to a mainHST 50, the output of which is transmitted to rear wheels serving assteeling wheels and to the non-steerable wheels, i.e. front wheels asshown in FIG. 1. The apparatus comprises a front wheel drive shaft and arear wheel drive shaft for outputting a front wheel drive force and rearwheel drive force respectively, a transmission unit for transmitting theoutput of the main HST to the front wheel drive axle and the rear wheeldrive axle, and a differential unit for reducing the rotational speed ofthe front wheel drive axle and increasing the rotational speed of therear wheel drive axle according to the steeling angle of the rear wheelswhen the rear wheels are turned. Indicated at 75 and 76 in the drawingare a front axle and a rear axle, respectively. Indicated at 70 is arear wheel drive unit connected to the rear axle 76 by an axle couplingfor outputting a rear axle drive force. Further indicated at 80 is a PTOunit for delivering a drive force to a mower or the like. Indicated at31 is a differential HST, and at 32 and 35 are a hydraulic pump andhydraulic motor, respectively, of the differential HST.

First, the main HST 50 will be described. FIG. 2 is a plan view insection of the main HST and the surroundings thereof. The main HST 50has a hydraulic pump 52 and a hydraulic motor 55 at least one of whichis of the variable displacement type, and a housing 51 for enclosing thepump and the motor. According to the present embodiment, the hydraulicpump 52 is of the variable displacement type, and the hydraulic motor 55is a fixed displacement motor. The housing 51 has a body 51 a and anextension 51 b extending outward from the body 51 a widthwise of thevehicle. The pump 52 and the motor 55 are accommodated in the housingbody 51 a, while the PTO unit 80 is enclosed in the housing extension 51b.

The housing body 51 a is open at its front side and closed with a rearwall at its rear side, while the extension 51 b is closed with a frontwall at its front side and open at its rear side. The front opening ofthe body 51 a is closed with a hydraulic block 85, and the rear openingof the extension 51 b is closed with a closure member 86. This resultsin the following advantage. If the body 51 a and the extension 51 b areboth open at the front side in the housing comprising these components51 a, 51 b, there arises a need to lengthen the block 85 widthwise ofthe vehicle body for closing the front opening to entail a cost increaseowing to an increase in the amount of material. Since the hydraulicblock 85 needs to have a considerable thickness to ensure a quantity ofoil, the increase in the length of the block 85 widthwise of the vehiclerequires an increased amount of material. When the housing 51 has theconstruction described, the mold for the body 51 a having the frontopening can be removed toward the front, and the mold for the extension51 b having the rear opening is removable rearward, with the result thatthe housing can be cast easily.

The hydraulic pump 52 has a pump shaft 53 extending longitudinally ofthe vehicle body. The pump shaft 53 has a rear end projecting rearwardfrom the housing 51 and connected to the output shaft of the drivesource 60 (see FIG. 1). The pump shaft 53 has a front end extendingforward from the housing 51 and projecting forward through the block 85.A gear 54 is supported on the pump shaft 53 at the portion thereof tothe rear of the body of the hydraulic pump nonrotatably relative to theshaft. The gear 54 is coupled to a drive member 81 of the PTO unit 80.The PTO unit 80 is on/off-controlled by a hydraulic clutch 83 comprisingthe drive member 81 and a driven member 82.

On the other hand, the hydraulic motor 55 has a motor shaft 56 providedapproximately in parallel to the pump shaft 53. The motor shaft 56 has afront end extending forward from the housing 51 and projecting forwardthrough the hydraulic block 85.

Next, a description will be given of the front wheel drive shaft 5, rearwheel drive shaft 6 and transmission unit 10 of the driving apparatus 1according to the present invention. FIG. 3 is a cross sectional view indevelopment of the front wheel drive shaft 5, rear wheel drive shaft 6,transmission unit 10, and the surroundings of these components, and FIG.4 is a view in section taken along the line A—A in FIG. 1.

With reference to FIG. 3, the front wheel drive shaft 5 and the rearwheel drive shaft 6 are arranged on approximately the same axislongitudinally of the vehicle and spaced apart from each other at theiropposed ends. More specifically, a predetermined space is providedbetween the rear end face of the shaft 5 and the front end face of theshaft 6. A differential gear unit 65 for driving the front axle 75 isconnected to the front end of the front wheel drive shaft 5, while theaforementioned rear wheel drive unit 70 is connected to the rear end ofthe rear wheel drive shaft 6.

The transmission unit 10 comprises a main drive shaft 11 disposed in thespace between the front wheel drive shaft 5 and the rear wheel driveshaft 6 on the same axis as these shafts, a front wheel planetary gearunit 20 interconnecting the main drive shaft 11 and the front wheeldrive shaft 5, a rear wheel planetary gear unit 20′ interconnecting themain drive shaft 11 and the rear wheel drive shaft 6, a driving powertransmission mechanism 13 interconnecting the hydraulic motor shaft 56of the main HST and the main drive shaft 11, and a casing 12 housingthese components.

With reference to FIGS. 3 and 4, the front wheel planetary gear unit 20comprises a sun gear 21 mounted on the main drive shaft 11, an outerwheel 23 having an inner gear 22 surrounding the sun gear 21, aplurality of planetary gears 24 meshing with both the sun gear 21 andthe inner gear 22, and a carrier 25 supported by the front wheel driveshaft 5 nonrotatably relative thereto. By a pin 26, the carrier 25 ismade to rotate with the revolution of the planetary gears 24 around thesun gear 21. Further a gear 27 is provided on the outer periphery of theouter wheel 23. On the other hand, the rear wheel planetary gear unit20′ also comprises the same components as the unit 20, i.e., a sun gear21′, an outer wheel 23′, planetary gears 24′, a carrier 25′ and a gear27′.

According to the present embodiment, the driving power transmissionmechanism 13 is a power transmission gear mechanism which comprises agear 13 a mounted on the motor shaft 56 of the main HST 50 nonrotatablyrelative thereto, an intermediate gear 13 b meshing with the gear 13 a,an idle shaft 13 c supporting the intermediate gear 13 b thereonrotatably relative thereto, a gear 13 d meshing with the intermediategear 13 b, a support shaft 13 e supporting the gear 13 d thereonnonrotatably relative thereto, a gear 13 f mounted on the support shaft13 e nonrotatably relative thereto, and a gear 13 g meshing with thegear 13 f and supported on the main drive shaft 11 nonrotatably relativethereto, whereby an improved transmission efficiency is achieved.Although the transmission mechanism 13 can be of the belt type, it isthen likely that the mechanism will be impaired in transmissionefficiency, for example, owing to slippage of the belt, whereas such adrawback can be effectively precluded with the mechanism of the presentembodiment.

The differential unit 30 will be described next which is included in thedriving apparatus of the invention. FIG. 5 is a development in verticalsection of the differential unit 30 and the vicinity thereof. Thedifferential unit 30 comprises a differential HST 31 having a hydraulicpump 32 and a hydraulic motor 35 at least one of which is of thevariable displacement type. According to the present embodiment, thehydraulic pump 32 is of the variable displacement type, and thehydraulic motor 35 is a fixed displacement motor. As shown in FIGS. 1and 2, the hydraulic pump 32 is disposed in front of the hydraulic block85 and has a pump shaft provided by a forwardly projecting portion ofthe pump shaft 53 of the main HST 50. Indicated at 34 in FIG. 2 is amovable swash plate for controlling the output of the hydraulic motor 35by varying the discharge rate of the hydraulic pump 32. On the otherhand, the hydraulic motor 35 is disposed in parallel to the main HST 50,in the rear of the casing of the differential gear unit 65 as seen inFIGS. 1, 4 and 5.

The differential unit 30 further comprises a differential powertransmission mechanism 40 for delivering the rotational output of thehydraulic motor 35 of the differential HST 31 to the front wheelplanetary gear unit 20 and the rear wheel planetary gear unit 20′therethrough. The power transmission mechanism 40 is adapted to give theouter wheel 23′ of the rear wheel planetary gear unit 20′ and the outerwheel 23 of the front wheel planetary gear unit 20 additional rotationsin opposite directions to each other. According to the presentembodiment, the mechanism 40 comprises, as shown in FIGS. 4 and 5, agear 40 a mounted on the motor shaft 36 of the differential HST 31nonrotatably relative thereto, an intermediate gear 40 b meshing withthe gear 40 a, an idle shaft 40 c supporting the intermediate gear 40 bthereon rotatably relative thereto, a gear 40 d meshing with theintermediate gear 40 b, a support shaft 40 e supporting the gear 40 dthereon nonrotatably relative thereto, and a gear 40 f mounted on thesupport shaft 40 e nonrotatably relative thereto. The mechanism 40further comprises a front wheel transmission member 41 for delivering arotation of the same direction as the rotation of the gear 40 f to theouter wheel 23 of the front wheel planetary gear unit 20, and a rearwheel transmission member 41′ for delivering a rotation opposite indirection to the rotation of the gear 40 f to the outer wheel 23′ of therear wheel planetary gear unit 20′. The front wheel transmission member41 comprises a gear member 41 a meshing with both the gear 40 f and theouter wheel 23 of the front wheel planetary gear unit 20. On the otherhand, the rear wheel transmission member 41′ comprises an idle gear 41b′, an idle shaft 41 c′ supporting the idle gear 41 b′ thereon rotatablyrelative thereto, and a gear member 41 a′ meshing with both the idlegear 41 b′ and the outer wheel 23′ of the rear wheel planetary gear unit20′.

The differential unit 30 is further provided with a link mechanism 45for operatively connecting the movable swash plate 34 of the hydraulicpump 32 of the differential HST 31 to a steering wheel 90 at thedriver's seat. FIG. 6 shows the steering wheel 90 and the rear wheels 8as coupled thereto. Indicated at 46 in the drawing is a rack-and-pinionassembly. FIGS. 7 to 9 schematically show the link mechanism 45. FIG. 7shows the state of the link mechanism 45 when the vehicle is advancedstraight. FIGS. 8 and 9 show the states of the link mechanism 45 whenthe vehicle is turned to the left and when the vehicle is turned to theright, respectively.

With reference to FIGS. 7 to 9, the link mechanism 45 comprises therack-and-pinion assembly 46, a pivotal plate 47 movable about a pivot 48a in operative relation with the assembly 46, and a connector 49 forconnecting the pivotal plate 47 to the movable swash plate 34.

The rack-and-pinion assembly 45 comprises a pinion 46 a fixed to therotary shaft of the steering wheel 90, and a pair of racks 46 b, 46 carranged at opposite sides of the pinion 46 a. In operative relationwith the rotation of the steering wheel, one of the racks is slidabletoward the pivotal plate, and the other rack is slidable in the oppositedirection. As shown in FIG. 7, the pair of racks 46 b, 46 c are soadapted that the corresponding rack ends are in the same position whenthe vehicle is advanced straight, that is, when the steering wheel 90 isnot rotated. The pair of racks 46 b, 46 c are arranged at differentpositions with respect to the direction of rotary shaft of the pinion 46a and are therefore unlikely to come into contact with each other. Thepivotal plate 47 is biased toward the pair of racks 46 b, 46 c by abiasing member 48 b at all times. The connector 49 is so connected tothe swash plate 34 as not to incline the swash plate 34 when the vehicleis advanced straight, namely, when the pivotal plate 47 is in theposition shown in FIG. 7 and as to incline the swash plate 34 when thevehicle is turned, namely when the pivotal plate is in the positionshown in FIG. 8 or 9.

The link mechanism 45 operates in the following manner. When the vehicleis being advanced straight without rotating the steering wheel 90, thepair of racks 46 b, 46 c do not push the pivotal plate 47. The pivotalplate 47 in this state does not incline the movable swash plate 34 asstated above. Accordingly, the motor shaft 36 of the differential HST 31is held out of rotation during the straight advance of the vehicle. Ifthe vehicle is turned leftward or rightward, i.e., when the steeringwheel 90 is rotated counterclockwise or clockwise, one of the racks 46b, 46 c pushes the pivotal plate 47 as shown in FIG. 8 or 9, whereby thepivotal plate 47 is moved about the pivot 48 a to incline the swashplate 34. Accordingly, the motor shaft 36 of the differential HST 31rotates when the vehicle is turned. Moreover, the inclination of theswash plate 34 is in proportion to the angle through which the pivotalplate 47 is pivotally moved, and the angle of pivotal movement of thepivotal plate 47 is proportional to the rotation angle of the steeringwheel 90, so that the rotational speed of the motor shaft 36 of thedifferential HST 31 varies in proportion to the rotation angle of thesteering wheel 90, i.e., to the steering angle of the rear wheels 8. Itis desired to incline the end portions of the racks 46 b, 46 c to bebrought into contact with the pivotal plate 47 as shown in FIGS. 7 to 9so that the position of contact of the rack 46 b with the plate 47 andthe position of contact of the other rack 46 c with the plate 47 will beat equal distances from the axis of the pivot 48 a. The angle of pivotalmovement of the pivotal plate 47 by the sliding movement of the rack 46b can then be made equal to the angle of pivotal movement of the plate47 by the movement of the other rack 46 c.

The operation of the driving apparatus 1 thus constructed will bedescribed below. FIG. 10 is a diagram of the power transmission systemof the vehicle 100 comprising the driving apparatus 1.

The transmission unit 10 for transmitting the rotational output of themain HST 50 will be described first. The rotational output of the motorshaft 56 of the main HST 50 is transmitted by the driving powertransmission mechanism 13 to the main drive shaft 11, which has fittedtherearound the sun gear 21 of the front wheel planetary gear unit 20and the sun gear 21′ of the rear wheel planetary gear unit 20′.Accordingly, the sun gears 21, 21′ are rotated at the same speed in thesame direction.

The differential unit 30 will be described next. When the vehicle isadvanced straight, that is, when the steering wheel 90 is not rotated,the differential unit 30 operates in the following manner. When thevehicle is advanced straight, the motor shaft 36 of the differential HST31 does not rotate as previously stated. Consequently, the outer wheels23, 23′ coupled to the motor shaft 36 by the differential powertransmission mechanism 40 are held out of rotation. Thus, the innergears 22, 22′ of the planetary gear units 20, 20′ are held fixed whenthe vehicle is advanced straight. On the other hand, the sun gears 24,24′ are held in rotation at the same speed in the same direction by therotational output of the main HST 50 as previously stated. Accordingly,the planetary gears 24, 24′ revolve at the same speed in the samedirection, causing the pins 26, 26′ to rotate the respective carriers25, 25′ at the same speed in the same direction. The front wheel driveshaft 5 and the rear wheel drive shaft 6 are rotated at the same speedin the same direction, rotating the front and rear wheels 7, 8 at thesame speed in the same direction.

Now, the gear ratio between the sun gears 21, 21′ and the carrier 25,25′ will be considered. Suppose the number of teeth of each of the sungears 21, 21′ is Za, the number of teeth of each of the planetary gears24, 24′ is Zb, and the number of teeth of each of the inner gears 22,22′ is Zc. Assuming that the carriers 25, 25′ rotate Nd turns when thesun gears 21, 21′ are rotated Na turns, the number of turns Nc of theouter wheels 23, 23′ is expressed by Equation (1) in Table 1.

TABLE 1 Sun gear Planetary gear Outer wheel Carrier 21, 21′ 24, 24′ 23,23′ 25, 25′ All Nd Nd Nd Nd fixed Carrier Na − Nd −(Na − Nd)* −(Na −Nd)* 0 fixed (Za/Zb) (Za/Zb)* (Zb/Zc) Combined Na Nb = Nc = Nd speed Nd− (Na − Nd)* Nd − (Na − Nd)* (Za/Zb) (Za/Zc) Equation(1)

Equation (1) can be modified as

Nd=(Za*Na+Nc)/(Za+Zc)  Equation (2)

When the vehicle is advanced straight, the inner gears 22, 22′ are inthe fixed state as previously stated, that is, the outer wheels 23, 23′are held fixed, so that Nc=0. The number of revolutions Nd of thecarriers is given by

Nd=(Za*Na)/(Za+Zc)  Equation (3)

Next, a description will be given of the operation of the differentialunit 30 when the vehicle is turned, namely, when the steering wheel 90is rotated counterclockwise or clockwise. When the vehicle is turned,the motor shaft 36 of the differential HST 31 rotates at a speed inaccordance with the rotation angle of the steering wheel 90 as alreadydescribed. The rotational output of the motor shaft 36 of thedifferential HST 31 is delivered through the differential powertransmission mechanism 40 to the outer wheel 23 of the front wheelplanetary gear unit 20 and the outer wheel 23′ of the rear wheelplanetary gear unit 20′ as additional rotations in directions oppositeto each other.

As will be apparent from Equation (2), therefore, the carrier 25′ of thegear unit 20′ is rotated at an increased speed while the carrier 25 ofthe gear unit 20 is rotated at a decreased speed when the differentialpower transmission mechanism 40 is so designed as to rotate the outerwheel 23′ of the gear unit 20′ in the same direction as the rotation ofthe sun gear 21′ and to rotate the outer wheel 23 of the gear unit 20 inopposite direction to the rotation of the sun gear 21. Consequently, therear wheel drive shaft 6 rotates at a higher speed than when advancingthe vehicle straight, and the front wheel drive shaft 5 rotates at alower speed than when advancing the vehicle straight, causing the rearwheels 8 to rotate at an increased speed and the front wheels 7 torotate at a reduced speed.

Thus, the driving apparatus 1 according to the present embodiment isadapted to rotate the steerable wheels and the nonsteerable wheels atthe same speed when advancing the vehicle straight and to rotate thesteerable wheels, i.e., the rear wheels 8, at an increased speed and thenonsteerable wheels, i.e., the front wheels 7, at a reduced speed whenturning the vehicle. This effectively obviates the skids of thesteerable wheel owing to the difference in turning radius between thesteerable wheel and the nonsteerable wheel during turning of thevehicle.

Furthermore, the driving apparatus 1 is adapted to drive the steerablewheels at an increased speed and the nonsteerable wheels at a decreasedspeed and therefore provides a capability of a great rotational speeddifference between the steerable wheels and the nonsteerable wheels.Accordingly, even if a great difference occurs in turning radius betweenthe steerable wheel and the nonsteerable wheel by a sharp turn of thevehicle, a speed difference is readily available to offset the greatdifference in turning radius.

The driving apparatus 1 is so designed as to vary the speed increasingratio of the steerable wheels and the reduction ratio of thenonsteerable wheels by varying the speed of the motor shaft 36 of thedifferential HST in proportion to the rotation angle of the steeringwheel 90. It is therefore possible to produce a rotational speeddifference between the steerable wheel and the nonsteerable wheel inaccordance with the turning radius difference therebetween which variesin proportion to the steering angle of the steering wheel.

With the conventional driving apparatus which gives a definiterotational speed to the nonsteerable wheels and gives the steerablewheels only two different rotational speeds, i.e., the same speed as thenonsteerable wheels and a speed higher than this speed, the rotationalspeed difference between the steerable wheel and the nonsteerable wheelis constant regardless of the steering angle of the steering wheel,whereas the difference in turning radius between the steerable wheel andthe nonsteerable wheel varies with the steering angle of the steeringwheel. The conventional apparatus is therefore unable to effectivelypreclude the steerable wheel from skidding.

With the driving apparatus 1 of the present embodiment, on the otherhand, the rotational speed difference between the steerable wheel andthe nonsteerable wheel is variable with the steering angle of thesteering wheel, so that the skid of the steerable wheel can be preventedeffectively in accordance with the steering angle of the steering wheel.

Although the present embodiment has been described with reference to avehicle wherein the rear wheels are steerable wheels and the frontwheels are nonsteerable wheels, the invention is not limited to suchvehicles but is of course applicable also to vehicles wherein the frontwheels serve for steering, and the rear wheels are nonsteerable wheels.

FIG. 12 shows another embodiment of the driving apparatus of the presentinvention. Throughout the drawings showing the first and secondembodiments, like parts are designated by like reference numerals andwill not be described in detail repeatedly. The driving apparatus, whichhas basically the same construction as the foregoing embodiment,comprises a transmission unit 10′ for receiving the rotational output ofthe main HST 50, and a differential unit 30′ for adjusting the rotationof the steerable wheel drive shaft 5 and the nonsteerable wheel driveshaft 6 according to the steering angle of the steering wheel. As in thefirst embodiment, the steerable wheel drive shaft 5 and the nonsteerablewheel drive shaft 6 are arranged on approximately the same axis andspaced apart from each other at opposed ends thereof. The transmissionunit 10′ comprises a first differential gear mechanism 110 having a pairof first sun bevel gears 115, 116 fixedly mounted on the steerable wheeldrive shaft 5 and the nonsteerable wheel drive shaft 6, respectively, attheir opposed ends. The first differential gear mechanism 110 comprisesa first casing 111 covering the pair of first sun gears 115, 116 androtatably supported on the drive shafts 5, 6, a first ring gear 112provided externally on the first casing 111 for receiving the rotationaloutput from the main HST 50, and a plurality of first planetary bevelgears 114 meshing with the pair of first sun gears 115, 116 androtatably supported by a shaft 113 fixedly provided inside the firstcasing 111 and extending diametrically of the casing.

The differential unit 30′ comprises a differential HST 31 for receivingpower from the drive source and outputting a differential rotationaldrive force, a second differential gear mechanism 130 for receiving therotational output from the differential HST, and an adjustingtransmission mechanism for transmitting an output from the seconddifferential gear mechanism to the steerable wheel drive shaft 5 and thenonsteerable wheel drive shaft 6. The second differential gear mechanism130 comprises a pair of rotary shafts 137, 138 supported by the housingof the driving apparatus, arranged on approximately the same axis andspaced apart from each other at opposed ends thereof, a pair of secondsun bevel gears 135, 136 fixedly mounted on the respective rotary shaftsat their opposed ends, a second casing 131 covering the pair of secondsun gears and rotatably supported on the pair of rotary shafts, a secondring gear 132 provided externally on the second casing for receiving therotational output from the differential HST 31, and a plurality ofsecond planetary bevel gears 134 meshing with the pair of second sungears and rotatably supported by a shaft 133 fixedly provided inside thesecond casing 131 and extending diametrically of the second casing.

The adjusting transmission mechanism comprises two gear trains 140, 150each comprising a drive gear 142 (154) fixed to the rotary shafts 137(138), and a driven gear 141 (151) fixed to the steerable wheel driveshaft 5 (the nonsteerable wheel drive shaft 6). At least one of the twogear trains has at least one intermediate gear so as to render the twodriven gears 141, 151 rotatable in directions different from each other.With the present embodiment, the gear train 150 has one intermediategear 153 mounted on a shaft 152. In the gear trains 140, 150, the gears141, 142 are equal in diameter, and the gears 151, 154 are equal indiameter.

The driving apparatus shown in FIG. 12 operates in the following manner.The output of motor shaft 56 of the main HST 50 is delivered via thedriving power transmission mechanism 13 to the ring gear 112. Therotation of the ring gear 112 is transmitted to the steerable wheeldrive shaft 5 and the nonsteerable wheel drive shaft 6 by way of thefirst casing 111, the planetary gears 113 and the pair of sun gears 115,116. The shafts 5, 6 have their rotational speed altered by the actionof the two gear trains 140, 150 of the adjusting transmission mechanismin the following manner.

When the vehicle is advanced straight, the motor shaft 36 of thedifferential HST 31 does not rotate as in the first embodiment.Accordingly, the ring gear 132 and the second casing 131 are held out ofrotation which are coupled to the motor shaft 36 via the differentialpower transmission mechanism 40. As a result, the rotary shafts 137, 138of the second differential gear mechanism 130 are rotated at the samespeed by the sun gears 135, 136 and the planetary gears 134. The gears141 and 151 are therefore rotated also at the same speed. Consequently,the drive shafts 5, 6 connected to the first differential gear mechanism110 are also rotated at the same speed, thus driving the steerablewheels and the nonsteerable wheels at the same speed.

When the vehicle is turned, on the other hand, the motor shaft 36 of thedifferential HST 31 rotates at a speed in accordance with the rotationangle of the steering wheel in the driver's seat as described in thefirst embodiment. The output of the motor shaft 36 rotates the ring gear132 of the second differential gear mechanism 130 via the differentialpower transmission mechanism 40. The second casing 131 thereforerotates, consequently increasing or reducing the number of revolutionsof the shafts 137, 138 by a quantity equal to the number of revolutionsof the second casing.

The increase or reduction in the number of revolutions of the shafts137, 138 is transmitted in the following manner. The two gear trains140, 150 are so adapted that the two driven gears 141, 151 rotate indirections different from each other as previously described. Indiameter, moreover, the gears 141, 142 are equal, and the gears 151, 154are equal. Accordingly, when the rotary shafts 137, 138 rotate at aspeed increased by the same number of revolutions, one of the gears 141,151 increases and the other gear reduces in speed by the same number ofrevolutions.

When the differential power transmission mechanism 40 is so adapted asto give an increased speed to the gear 141 and a reduced speed to thegear 151 by the rotation of the motor shaft 36 of the differential HST31 when the vehicle is turned, the steerable wheel drive shaft 5 istherefore rotated at an increased speed and the nonsteerable wheel driveshaft 6 at a reduced speed for turning the vehicle. This effectivelyprecludes the skids of the steerable wheels due to the difference inturning radius between the steerable wheel and the nonsteerable wheelduring turning of the vehicle in the vehicle driving apparatus shown inFIG. 12 as in the first embodiment. According to the present embodiment,the function described above is available by making the first and seconddifferential gear mechanisms 110, 130 identical in construction andattaching these mechanisms, as oriented in opposite directions, to thehousing of the driving apparatus. This permits use of common componentsfor these two mechanisms, consequently leading to a reduction inmanufacturing cost.

We claim:
 1. A driving apparatus for a vehicle for transmitting powerfrom a drive source installed in a body of the vehicle to steerablewheels and nonsteerable wheels via a main HST, the driving apparatusbeing characterized in that the driving apparatus comprises: atransmission unit for receiving a rotational output from the main HST, asteerable wheel drive shaft and a nonsteerable wheel drive shaft forreceiving a rotational output from the transmission unit andtransmitting the rotational output respectively to an axle for drivingthe steerable wheels and an axle for driving the nonsteerable wheels,and a differential unit for increasing a rotational speed of thesteerable wheel drive shaft and decreasing a rotational speed of thenonsteerable wheel drive shaft proportionally according to a steeringangle of a steering wheel to be handled by a driver, the transmissionunit comprising a main drive shaft disposed between the steerable wheeldrive shaft and the nonsteerable wheel drive shaft, a driving powertransmission mechanism for transmitting the rotational output of themain HST to the main drive shaft, and a steerable wheel planetary gearunit and a nonsteerable wheel planetary gear unit for transmittingrotation of the main drive shaft respectively to the steerable wheeldrive shaft and the nonsteerable wheel drive shaft so as to rotate thewheel drive shafts in a same direction, and the differential unitcomprising a differential HST for receiving the power from the drivesource and outputting an adjusted rotational drive force proportionallyaccording to the steering angle of the steering wheel, and adifferential power transmission mechanism for receiving the rotationaloutput from the differential HST and giving mutually opposite additionalrotations respectively to the steerable wheel planetary gear unit andthe nonsteerable wheel planetary gear unit to cause the increasedrotational speed of the steerable wheel drive shaft and the decreasedrotational speed of the nonsteerable wheel drive shaft.
 2. A drivingapparatus for a vehicle for transmitting power from a drive sourceinstalled in a body of the vehicle to steerable wheels and nonsteerablewheels via a main HST, the driving apparatus being characterized in thatthe driving apparatus comprises: a transmission unit for receiving arotational output from the main HST, a steerable wheel drive shaft and anonsteerable wheel drive shaft for receiving a rotational output fromthe transmission unit and transmitting the rotational outputrespectively to an axle for driving the steerable wheels and an axle fordriving the nonsteerable wheels, and a differential unit having adifferential power transmission mechanism for i) rotating the steerablewheel drive shaft at a speed increased by a rate according to a steeringangle of a steering wheel to be handled by a driver and ii) rotating thenonsteerable wheel drive shaft at a speed decreased by said rate,wherein said driving apparatus is characterized in that the steerablewheel drive shaft drive shaft and the nonsteerable wheel drive shaft arearranged approximately on a common axis and spaced apart from each otherat opposed ends thereof, the transmission unit comprising: a main driveshaft disposed between the opposed ends of the steerable wheel driveshaft and the nonsteerable wheel drive shaft on the same axis as the twoshafts, a driving power transmission mechanism for transmitting fortransmitting the rotational output of the main HST to the main driveshaft, and a steerable wheel planetary gear unit and a nonsteerablewheel planetary gear unit for transmitting rotations of the main driveshaft respectively to the steerable wheel drive shaft and thenonsteerable wheel drive shaft so as to rotate the wheel drive shafts inone direction, the steerable wheel planetary gear unit having a firstsum gear mounted on the main drive shaft, a first outer wheelsurrounding the first sum gear, a first inner gear provided on an innerperiphery of the first outer wheel, first planetary gears arrangedbetween the first sun gear and the first inner gear, and a first carriersupported on the steerable wheel drive shaft nonrotatably relativethereto and rotatable with revolutions of the planetary gears, thenonsteerable wheel planetary gear unit having a second sun gear mountedon the main drive shaft, a second outer wheel surrounding the second sungear, a second inner gear provided on an inner periphery of the secondouter wheel, second planetary gears arranged between the second sun gearand the second inner gear, and a second carrier supported on thenonsteerable wheel drive shaft nonrotatably relative thereto androtatably with revolutions of the second planetary gear.
 3. A driveapparatus according to claim 2 which is characterized in that thedifferential unit comprises: a differential HST for receiving the powerfrom the drive source and outputting a rotational drive force, and adifferential power transmission mechanism for receiving the rotationaloutput from the differential HST and adjusting rotational speeds of thefirst outer wheel and the second outer wheel respectively in oppositedirections to each other, the differential HST being adapted not tooutput the rotational drive force when the vehicle is advanced straightand to output the rotational drive force with a number of revolutions inaccordance with the steering angle of the steering wheel when thevehicle is turned, the differential power transmission mechanism beingadapted to adjust the rotational speed of the first outer wheel in thesame direction as the rotational direction of the first sun gear rotatedby the main HST and to adjust the rotational speed of the second outerwheel in a direction opposite to the rotational direction of the secondsun gear rotated by the main HST.
 4. A driving apparatus according toclaim 3 which is characterized in that the differential HST has ahydraulic pump and a hydraulic motor, at least one of the hydraulic pumpor the hydraulic motor being variable in displacement by operating adisplacement altering member, the differential unit comprising a linkmechanism coupling a steering wheel to be handled by a driver to thedisplacement altering member, the link mechanism being adapted to reduceapproximately to zero a discharge rate of one of the hydraulic pump orthe hydraulic motor which has the displacement altering member when thesteering wheel is directed to advance the vehicle straight and toincrease the discharge rate as the steering angle of the steering wheelincreases when the steering wheel is directed to turn the vehicle.
 5. Adriving apparatus according to any one of claims 2 to 4 wherein thedriving power transmission mechanism and the differential powertransmission mechanism are each a power transmission gear mechanism. 6.A driving apparatus for a vehicle for transmitting power from a drivesource installed in a body of the vehicle to steerable wheels andnonsteerable wheels via a main HST, the driving apparatus beingcharacterized in that the driving apparatus comprises: a transmissionunit for receiving a rotational output from the main HST, a steerablewheel drive shaft and a nonsteerable wheel drive shaft for receiving arotational output from the transmission unit and transmitting therotational output respectively to an axle for driving the steerablewheels and an axle for driving the nonsteerable wheels, and adifferential unit having a differential power transmission mechanism fori) rotating the steerable wheel drive shaft at a speed increased by arate according to a steering angle of a steering wheel to be handled bya driver and ii) rotating the nonsteerable wheel drive shaft at a speeddecreased by said rate, wherein said driving apparatus is characterizedin that the steerable wheel drive shaft and the nonsteerable wheel driveshaft are arranged approximately on common axis and spaced apart fromeach other at opposed ends thereof, the transmission unit comprising afirst differential gear mechanism having a pair of first sun gearsfixedly mounted on each of the steerable wheel drive shaft and thenonsteerable wheel drive shaft at opposed ends thereof, the firstdifferential gear mechanism comprising a first casing covering the pairof first sun gears and rotatably supported on the drive shafts, a firstring gear provided externally on the first casing for receiving therotational output from the main HST, and a plurality of first planetarygears meshing with the pair of first sun gears and rotatably supportedby a shaft fixedly provided inside the first casing and extendingdiametrically of the casing, the differential unit comprising adifferential HST for receiving a power from the drive source andoutputting a rotational drive force, a second differential gearmechanism for receiving the rotational output from the differential HST,and an adjusting transmission mechanism for transmitting an output fromthe second differential gear mechanism to the steerable wheel driveshaft and the nonsteerable wheel drive shaft, the second differentialgear mechanism comprising pair of rotary shafts supported by a housingof the driving apparatus, arranged approximately on common axis andspaced apart from each other at opposed ends thereof, a pair of secondsun gears fixedly mounted on the respective rotary shafts at theiropposed end, a second casing covering the pair of second sun gears androtatably supported on the pair of rotary shafts, a second ring gearprovided externally on the second casing for receiving the rotationaloutput from the differential HST, and a plurality of second planetarygears meshing with the pair of second sun gears and rotatably supportedby a shaft fixedly provided inside the second casing and extendingdiametrically of the second casing, the adjusting transmission mechanismcomprising two gear trains provided for the steerable wheel drive shaftand the nonsteerable wheel drive shaft respectively and each comprisinga drive gear fixed to the rotary shaft, and a driven gear fixed to thecorresponding drive shaft, at least one of two gear trains having atleast one intermediate gear so as to render the two driven gearsrotatable in directions different from each other, the drive gear andthe driven gear of each of the gear trains being equal in diameter.
 7. Adriving apparatus for a vehicle for transmitting power via a main HSTfrom a drive source installed in a body of the vehicle to a first pairof driving wheels and a second pair of driving wheels, the pairs ofwheels being positioned at the front and rear of the vehicle body and atleast one pair of the pairs of driving wheels being steerable, thedriving apparatus being characterized in that the driving apparatuscomprising: a transmission unit for receiving a rotational output fromthe main HST, a first drive shaft and a second drive shaft for receivinga rotational output from the transmission unit and transmitting therotational output respectively to an axle for driving the first pair ofwheels and an axle for driving the second pair of wheels, and adifferential unit for increasing a rotational speed of the steerablewheel drive shaft and decreasing a rotational speed of the nonsteerablewheel drive shaft proportionally according to a steering angle of asteering wheel to be handled by a driver, the transmission unitcomprising a main drive shaft disposed between the steerable wheel driveshaft and the nonsteerable wheel drive shaft, a driving powertransmission mechanism for transmitting the rotational output of themain HST to the main drive shaft, and a steerable wheel planetary gearunit and a nonsteerable wheel planetary gear unit for transmittingrotation of the main drive shaft respectively to the steerable wheeldrive shaft and the nonsteerable wheel drive shaft so as to rotate thewheel drive shafts in a same direction, and the differential unitcomprising a differential HST for receiving the power from the drivesource and outputting an adjusted rotational drive force proportionallyaccording to the steering angle of the steering wheel, and adifferential power transmission mechanism for receiving the rotationaloutput from the differential HST and giving mutually opposite additionalrotations respectively to the steerable wheel planetary gear unit andthe nonsteerable wheel planetary gear unit to cause the increasedrotational speed of the steerable wheel drive shaft and the decreasedrotational speed of the nonsteerable wheel drive shaft.